Optimization of ultrasound-assisted extraction of phenolic compounds: Oleuropein, phenolic acids, phenolic alcohols and flavonoids from olive leaves and evaluation of its antioxidant activities

Olive leaves are well known for many useful pharmacological effects. Some of the health benefits are related to phenolic composition, especially to oleuropein (OLE) and flavonoids (FLs) content. This study aimed to investigate the influence of ultrasound-assisted extraction conditions (solvent type, solvent concentration, extraction time and temperature) on the extract yield of OLE, hydroxytyrosol (HTR), FLs (rutin, luteolin, luteolin-7-O-glucoside and apigenin) and phenolic acids (protocatechuic, 4-hydroxybenzoic, vanillic, p-coumaric and ferulic acids) from olive leaves using single factor experiments approach. The total phenolic compounds (TPC) and their antioxidant activity based on 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity (RSA) and ferric reducing antioxidant power (FRAP) were also evaluated. The highest yields of OLE and FLs, the major phenolics in olive leaves, were obtained with 50% acetone, at 60 degrees C for 10 min extraction. However, the yields of HTR and PAs increased when water was used as extraction solvent. Good, positive, correlation coefficients were found between OLE, TPC, RSA and FRAP in olive leaves, especially under the influence of solvent type and solvent concentration.